Answer:The question of when to take vitamins together or separately is an excellent one and which we address in the "What to Consider When Using" and "Concerns and Cautions" sections of our Reviews of vitamin or mineral supplements. How you take a supplement can be just as important as which product you take -- both may impact how much of a nutrient your body actually gets.
A few rules of thumb:
- If you take a large dose of a mineral, it will compete with other minerals to reduce their absorption. The mineral most often taken in large amounts is calcium: The dose is usually several hundred milligrams, compared to doses of just a few milligrams or even microgram amounts (1,000 micrograms = 1 milligram) of most other minerals. So if you take several hundred milligrams of calcium from a supplement, take it at a different time of day than other mineral supplements or a multivitamin/multimineral supplement. Doses of magnesium can also be relatively large and should, ideally, be taken apart from other minerals. If you take high doses of zinc long-term (50 mg or more per day for 10 weeks or longer ), be aware that it can cause copper deficiency, so you may need to supplement with copper as well.
- High doses of calcium or other minerals (including magnesium, certain forms of iron, and zinc) from supplements may decrease the absorption of carotenoids, such as beta-carotene, lycopene and astaxanthin, from foods and/or supplements. It is best to take carotenoid supplements at a different time of day than a supplement or meal containing large amounts of a mineral (e.g., hundreds of milligrams of calcium or magnesium).
- Some vitamins can actually enhance the absorption of other nutrients. Vitamin C, for example, can enhance iron absorption from supplements and plant foods.
- The fat-soluble vitamins (A, D, E, and K) are likely to be better-absorbed if taken with a meal that contains fats. In fact, one study found that taking vitamin D with dinner rather than breakfast increased blood levels of vitamin D by about 50%. However, evidence (mainly from animal and cell studies) suggests that moderate to large doses of fat-soluble vitamins reduce absorption of other fat-soluble vitamins - by about 10 to 50% - due to competition. Absorption of vitamin K appears to be particularly reduced by other fat-soluble vitamins, while vitamin A absorption is least affected and may actually be better absorbed when taken with vitamin E (Goncalves, Food Chem 2015). Taking vitamins D, E, or K several hours before or after other fat-soluble vitamins would seem to maximize their absorption.
- Taking certain supplements with food can reduce gastrointestinal side-effects. For example, taking magnesium with food can reduce the occurrence of diarrhea, and taking iron with food can reduce the chance of stomach upset.
- Be aware that vitamins and minerals can also affect the absorption and effectiveness of medications. You'll find more specific information about this in the "Concerns and Cautions" section of each of our Reviews.
Keep in mind that these issues are not of significant concern when consuming a multivitamin providing up to the recommended daily intakes (RDAs) of vitamins and minerals -- as long as it does not contain more than 250 mg of either calcium or magnesium.